This invention relates to a direct spark ignition system for post-mixed burners which reliably ignites the combustible mixture while avoiding high igniter wear as well as the need for complex igniter protection systems.
Burners are generally divided into two types, pre-mixed and post-mixed. A pre-mixed burner is one in which the fuel and the oxidant are mixed before they enter the burner nozzle and prior to being discharged into the combustion zone. A post-mixed burner is one in which the fuel and oxidant are kept separate until discharged into the combustion zone.
Ignition systems are customarily designed with reference primarily to two criteria: (1) reliable ignition of the fuel-oxidant mixture, and (2) protection of the ignition is achieved. It can be readily appreciated that the elements of an ignition system will be easily destroyed at the temperatures characteristic of a combustion zone.
A typical post-mixed burner ignition system normally comprises means to shield the ignition system from the high combustion zone temperatures since the ignition system must deliver the ignition flame to the fuel-oxidant mixture in the combustion zone. A commonly used means employs a separate pilot flame which is ignited in an area protected from the intense heat of the combustion zone and then passed to the combustion zone to ignite the main combustion components. The major disadvantage of such a system is the requirement of having a duplicate fuel and oxidant supply system attached to the main burner assembly.
Another typical post-mixed burner ignition system is one that retracts the ignition system immediately after the delivery of the ignition flame. Such means are mechanically complicated and require high initial capital costs as well as high operating and maintenance costs.
Still another typical post-mixed burner ignition system is one which employs means to create good fuel-oxidant mixing in the area of the spark. As mentioned previously, a post-mixed burner is one where fuel and oxidant are not mixed until they are discharged into the combustion zone. Such post-mixed burners promote good mixing of fuel and oxidant in the area of the spark in place of providing sparks to the area of good mixing, as with a retraction device. Disadvantages of this system include the need for a good-mixing promoter, such as a deflection device, atomizer, etc., which may be bulky or otherwise cumbersome, and the fact that spark electrode wear is markedly increased when burning occurs near it, as happens when good fuel-oxidant mixing occurs in its vicinity.
Where the ignition system is not a direct system, such as an intermittent or interrupted pilot flame, burning near the electrode may be tolerable, because many systems are not designed to be fired continuously. Thus, these systems are able to tolerate momentary high temperatures around the electrode caused by burning of the well-mixed fuel oxidant mixture in their proximity. A direct ignition system which is required to be fired continuously cannot tolerate such high temperatures near the electrode without incurring high wear or deterioration.
Still another typical post-mixed burner ignition system provides sparks to an area of good fuel-oxidant mixing without placing the spark generation system in that area by projecting only the spark into the area. This may be done by increasing the voltage used to produce the spark so that the spark loops outward from the generation system into the area of good mixing: alternatively, the spark may be made to loop outward by placing it in the path of a swiftly moving gas stream. As can be appreciated, methods such as these require a significant increase in energy usage.
An ignition system for a post-mixed burner which is capable of providing ignition reliability, while affording protection for the ignition system from the hot combustion zone conditions, while avoiding the need for additional parts to the burner assembly and high energy requirements to effect ignition would be highly desirable.